1. Field of the Invention
The disclosure relates generally to methods and systems for drilling boreholes for the ultimate recovery of oil, gas or minerals. More particularly, the disclosure relates to methods and systems for avoiding, disrupting, and/or preemptively preventing undesirable “steady state” conditions and harmonic motions during drilling operations.
2. Background of the Technology
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached to a drillstring. The drill bit is typically mounted on the lower end of the drillstring as part of a bottomhole assembly (BHA) and is rotated by rotating the drillstring at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drillstring, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a path toward a target zone.
To aid in the removal of drilling cuttings from the bottom of the borehole, pressurized drilling fluid (commonly know as “mud” or “drilling fluid”) is pumped down the drillstring to the drill bit mounted at the lower end of the bottomhole assembly. The drilling fluid exits the drill bit through nozzles or jet assemblies positioned in bores formed in the body of the bit. To efficiently remove cuttings from the borehole, the drilling fluid must carry the cuttings radially outward on the borehole bottom, and then upward through the annulus between the drillstring and the borehole wall. As the drilling fluid flows past the cutting structure, the fluid impacts the borehole bottom and spreads radially outward to the annulus. In general, as the efficiency of the cutting removal is increased, the cutting efficiency and associated rate-of-penetration (ROP) of the drill bit are also increased.
A number of downhole devices placed in close proximity to the drill bit measure certain downhole parameters associated with the drilling and downhole conditions. Such devices typically include sensors for measuring downhole temperatures and pressures, azimuth and inclination measuring devices, and a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional downhole instruments, known as logging-while-drilling (“LWD”) and/or measurement-while drilling (“MWD”) tools, are frequently attached to the drillstring to determine the formation geology and formation fluid conditions during the drilling operations. The information provided to the operator during drilling usually includes drilling parameters, such as weight-on-bit (WOB), rotational speed of the drill bit and/or the drillstring, and the drilling fluid flow rate. In some cases, the drilling operator is also provided selected information from the downhole sensors such as bit location and direction of travel, downhole pressure, and possibly formation parameters such as resistivity and porosity.
Boreholes are usually drilled along predetermined paths and the drilling of a typical borehole proceeds through various formations. The downhole operating conditions may change and the operator must react to such changes and adjust the surface-controlled parameters to optimize the drilling operations. The drilling parameters typically controlled by the drilling operator to optimize the drilling operations include the weight-on-bit (WOB), drilling fluid flow through the drill pipe (flow rate and pressure), the drillstring rotational speed, axial position of the drillstring and drill bit within the borehole, and the density and viscosity of the drilling fluid. During most conventional drilling operations, the drilling operator adjusts the various surface-controlled drilling parameters in response to, or after, detection of certain downhole conditions.
In general, the drillstring, drill bit, and drilling fluid each input energy into the drilling process. Namely, rotation of the drillstring and drill bit input energy into the drilling process, the axial movement of the drillstring and the drill bit input energy into the drilling process, and the drilling fluid pressure and flow rate input energy into the drilling process. When the energy input by (a) the rotation of the drillstring and drill bit, (b) the flow of drilling fluid, (c) the movement of the drillstring and drill bit, or (d) the combination of (a) thru (c) is uniform and constant over a period of time, it has the potential to create undesirable “steady state” downhole conditions and/or harmonic motions, which may lead to common issues such as stick-slip, insufficient hole cleaning, bit whirl, drill-string whirl, excessive vibrations (lateral and/or axial), or combinations thereof.
As described above, during most conventional drilling operations, the drilling operator adjusts the various surface-controlled drilling parameters in response to, or after, detection of certain undesirable downhole conditions. Usually, the drilling operator monitors the downhole conditions, attempts to identify the occurrence of undesirable downhole conditions, and then takes action at the surface, by adjusting one or more of the surface-controlled drilling parameters, to disrupt the undesirable downhole condition(s). Accordingly, this conventional approach seeks to manually address the downhole issues after they arise. In some cases, by the time the drilling operator has recognized the downhole problem and altered the surface-controlled drilling parameters, damage to the drillstring, the drill bit, and/or other downhole components has already occurred.
Some drilling operations employ predictive models that receive data relating to surface and/or downhole conditions and output a set of recommended values for the drilling parameters (e.g., bit RPM) based on analysis of such measurements. The recommended drilling parameters may be implemented manually or via an automated control systems. However, the physics behind such modeling schemes is complex, and typically depend on accurate measurements of surface and downhole conditions, which are often difficult to obtain in the harsh drilling environment. Consequently, some of the predictive models are less effective than desired.
Accordingly, there is a need in the art for drilling systems and methods that overcome the problems associated with the prior art systems. Such drilling systems and methods would be particularly well received if they offered the potential to proactively disrupt or avoid undesirable steady state conditions and downhole harmonic motions.